The atomic mass of caesium with atomic number of 55 and neutron of of 78 is 133 amu.
The atomic mass of an element is the sum of the proton number and the neutron number.
The proton and neutron contributes mostly to the weight of an atom. The proton and neutron are located in the nucleus an atom.
The atomic number of an element is the same as the proton number of an element. Therefore, since the atomic number of caesium is 55, the proton number is also 55.
The atomic mass of caesium can be computed as follows:
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